A Critical Review on Improving the Fatigue Life and Corrosion Properties of Magnesium Alloys via the Technique of Adding Different Elements

Magnesium is the eighth-most abundant element in the world and its alloys have a widespread application in various industries such as electronic and transport (i.e., air, land, and sea) engineering, due to their significant mechanical properties, excellent machinability, high strength to weight ratios, and low cost. Although monolithic Mg metal is known as the lightest industrial metal (magnesium density is 30% less than the density of the aluminum, and this unique property increases the attractiveness of its usage in the transportation industry), one of the significant limitations of magnesium, which affects on its applications in various industries, is very high reactivity of this metal (magnesium with an electronegativity of 31.1 can give electrons to almost all metals and corrodes quickly). To overcome this problem, scholars are trying to produce magnesium (Mg) alloys that are more resistant to a variety of loads and environmental conditions. In this regard, Mg alloys include well-known materials such as aluminum (Al), Zinc (Zn), Manganese (Mn), Silicon (Si), and Copper (Cu), etc., and their amount directly affects the properties of final products. In the present review paper, the authors attempted to present the latest achievements, methods, and influential factors (finish-rolling, pore defects, pH value, microstructure, and manufacturing processes, etc.) on the fatigue life and corrosion resistance of most significant Mg alloys, including AM50, AM60, AZ31, AZ61, AZ80, AZ91, ZK60, and WE43, under various conditions. The summarized results and practical hints presented in this paper can be very useful to enhance the reliability and quality of Mg-made structures

Effect of thermomechanical processing defects on fatigue and fracture behaviour of forged magnesium

The microstructural origins of premature fatigue failures were investigated on a variety of forged components manufactured from AZ80 and ZK60 magnesium, both at the test specimen level and the full-scale component level. Both stress and strain-controlled approaches were used to characterize the macroscopically defect-free forged material behaviour as well as with varying levels of defect intensities. The effect of thermomechanical processing defects due to forging of a industrially relevant full-scale component were characterized and quantified using a variety of techniques. The fracture initiation and early crack growth behaviour was deterministically traced back to a combination of various effects having both geometric and microstructural origins, including poor fusion during forging, entrainment of contaminants sub-surface, as well as other inhomogeneities in the thermomechanical processing history.             At the test specimen level, the fracture behaviour under both stress and strain controlled uniaxial loading was characterized for forged AZ80 Mg and a structure-property relationship was developed. The fracture surface morphology was quantitatively assessed revealing key features which characterize the presence and severity of intrinsic forging defects.  A significant degradation in fatigue performance was observed as a result of forging defects accelerating fracture initiation and early crack growth, up to 6 times reduction in life (relative to the defect free material) under constant amplitude fully reversed fatigue loading.             At the full-scale component level, the fatigue and fracture behaviour under combined structural loading was also characterized for a number of ZK60 forged components with varying levels of intrinsic thermomechanical processing defects. A novel in-situ non-contact approach (utilizing Digital-Image Correlation) was used as a screening test to establish the presence of these intrinsic defects and reliably predict their effect on the final fracture behaviour in an accelerated manner compared to conventional methods

Characterization and Modeling of Forged ZK60 Mg Alloys under Quasi-static and Fatigue Loadings

In response to impending changes to the environmental regulations on the vehicles’ gas consumption rate, the transportation sector is motivated toward the widespread adoption of lightweight materials in the manufacturing of its products. Magnesium (Mg) alloys, being the lightest commercial metals available in the industry, can play an integral role in this scope with offering huge mass saving comparing to aluminum and steel. With the development of multi-material vehicle architecture concept in the automotive industry philosophy, ultra-light materials such as Mg alloys should be exploited in the component in which they perform the best. For vehicle parts which are driven by fatigue, e.g., suspension control arm, wrought Mg is a suitable candidate as a substitute for the current structural metals, due to its excellent fatigue performance in addition to opening mass saving windows. Of the commercially available Mg alloys, ZK- series, and in particular, ZK60 Mg alloys, have shown superb mechanical properties and formability. Moreover, amongst several prevalent industrial manufacturing techniques, forging is of particular interest because it has shown its promise to produce components with complex geometry and high strength. However, the mechanical behavior of forged ZK60 has been largely unknown so far. The current research work has aimed to fill this gap by characterizing the mechanical behavior of forged ZK60. The focus has been to establish a link between the material, structure and performance. The discovery-level knowledge, established through this project, develops a better understanding of the fatigue performance of this alloy and provides a wealth of mechanical performance data on this material in order for industry to make better use of it in real-world applications. Initially, the effects of open-die forging on the mechanical behavior of cast ZK60 was studied. A partially recrystallized microstructure with sharp basal texture was developed in the material. Also, the porosity volume was dramatically reduced after the forging. As a result, the tensile yield strength and elongation were increased by 21% and 72%, respectively. Under cyclic loadings, the forged material exhibited a better response in both the low cycle fatigue (LCF) and high cycle fatigue (HCF) regimes in the light of its higher ductility and strength, and lower content of porosities and intermetallic particles, which can play as crack nucleation sites. The fracture surfaces of the samples tested at various strain amplitudes were analyzed using the scanning electron microscope (SEM), and different crack initiation mechanisms were identified. At low strain amplitudes, corresponding to the HCF regime, persistent slip bands (PSB) and second-phase intermetallic particles were defined as the major causes of crack initiation, whilst at high strain amplitudes, ascribed to the LCF lives, the interactions between twin-twin bands besides twin-dislocation were recognized as the key reasons for cracking. Next, the quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along different directions, namely, the extrusion direction (ED), the radial direction (RD), and 45° with respect to the extrusion direction (45°), in the scope of process-structure-property-performance relationships. In contrast to the asymmetric quasi-static behavior of extrusion direction, radial and 45° directions manifested symmetric responses. Also, ED samples showed higher strength compared to the other two directions. The strain-controlled fatigue performance of ZK60 extrusion was insensitive to the material direction in the LCF regime. However, in the HCF regime, ED displayed fatigue responses superior to the RD and 45°. The texture measurement indicated a sharp basal texture along ED. Also, microstructural analyses revealed binary microstructure, explaining the ED’s asymmetric behavior and higher strength comparing to the other directions. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for ED’s identical fatigue response to that of RD and 45 in the LCF regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twinned grains, whereas that in RD was dominated by slip bands. Lastly, Smith-Watson-Topper and Jahed-Varvani models were employed to predict the fatigue lives along all directions using a single set of material parameters. The energy-based model yielded acceptable predictions for ZK60 extrusion with anisotropic behavior. Finally, the multiaxial fatigue characteristics of as-extruded and close-die “I-beam” extruded-forged ZK60 were investigated. Quasi-static tension and shear tests in addition to uniaxial, pure shear, and multiaxial cyclic tests under variety of loading paths as well as texture measurements and microstructural analysis were delivered to characterize the material’s behavior and understand the effects of forging on the performance of the alloy. It was discovered that the imparted thermo-mechanical process modified the initial sharp basal texture in the flanges of the I-beam. Secondly, following forging, the microstructures in the two flanges of the I-beams were similar. Furthermore, following the quasi-static tests, it was revealed that the axial behavior of the forging was superior to that of the starting extruded material, whereas the shear responses were comparable. Multiaxial fatigue tests demonstrated that non-proportionality does not change the fatigue life tremendously; however, it does affect the shear response remarkably. It was concluded that at low shear strain amplitudes that the shear strain is accommodated by slipping, the multiaxial behavior is somewhat dominated by the axial component. The microstructure of both undeformed and deformed samples after 20% shear strain under quasi-static loading was studied using the EBSD technique. The deformed sample showed considerable amount of {101 ̅2} tensile twins in the microstructure; hence, the developed texture plays an integral role in the material’s shear behavior under large strains where appreciable extension twin occurs. Finally, an energy-based fatigue model was employed that effectively explains the different damage contributions by the axial and torsional loadings at different strain amplitudes, and accurately predicts the proportional and non-proportional multiaxial fatigue lives for both as-extruded and forged alloys

Mechanical Behavior and the Role of Deformation Twinning in Wrought Magnesium Alloys Investigated Using Neutron and Synchrotron X-ray Diffraction

The mechanical behaviors and the associated deformation mechanisms with a focus on extension twinning under monotonic and cyclic loadings are investigated using neutron and synchrotron diffractions in the wrought magnesium alloys, ZK60A and AZ31B. It has been demonstrated that the extension twinning plays significant roles in the mechanical behaviors. The significant tension-compression asymmetries and high anisotropies are observed. The tension-compression asymmetries are related to the twinning activation in one direction but not in the opposite direction. The high anisotropies are correlated with the initial texture distinction relative to the loading directions. The similar yielding stresses are noted irrespective of the strain direction and strain sign if the deformation is dominated by twinning, while they are usually different with respect to the loading conditions if the dislocation slip is dominant. The extension twinning under tension exhibits a similar behavior to that under compression, presumably due to the same Schmid stress introduced on the twinning plane along the twinning shear direction. However, the distribution of basal poles within the twins under tension is closely related to the initial texture, while they are always aligned with the compressive axis under compression. The low-cycle fatigue resistances follow the empirical Basquin and Coffin-Manson relationships with the texture dependency observed, related to the different activation sequences of twinning and detwinning involved, respectively, under tension and compression determined by the initial texture. Specifically, the post-detwinning deformation characteristics may be an important factor in understanding the texture dependency. The extension twinning is readily activated if an applied stress/strain supports c-axis extension of the hexagonal-close-packed (hcp) structure. The unique reorientation of the twins facilitates detwinning in the twinned areas during the subsequent strain/stress reversal. Therefore, the cyclic plastic deformation is dominated by the alternating twinning and detwinning, and the initial texture is recovered once detwinning capability is exhausted, concurrent with the disappearance of twin bands. In particular, detwinning occurs almost immediately in the twinned grains upon unloading, which is driven by the local tensile stress along their c-axes as a result of the stress redistribution between the soft- and hard-grain orientations due to the plastic anisotropy

Equal-Channel-Angular Processing (ECAP) of Materials: Experiment and Theory

Equal Channel-Angular Processing (ECAP), as a severe plastic deformation of metals and composites, is analyzed both theoretically - to describe the ECAP macromechanics - and experimentally - to obtain ultrafine-grained materials with new thermo-mechanical properties - with a focus on hexagonal-closed-packed (HCP) structures such as Mg alloys. Due to their obvious similarity to ECAP, the slip-line–field theories developed for orthogonal cutting are applied to the ECAP deformation for predicting the shear-strain spatial heterogeneities. A theoretical model for predicting the plastic-deformation zone in an ECAP-ed billet with a free surface is provided, and is validated experimentally. A shear-strain-mapping procedure was developed by decomposing the large deformation process into fine steps, and, by analyzing the partially-deformed billets, the strain maps captured the spatio-temporal evolutions of the ECAP-induced plastic shear strains. This approach was later generalized for studying the local behavior of different material parameters, such as textures (texture mapping). The mechanical testing of the as-received and ECAP-deformed Mg-alloys (ZK60 and AZ31) was performed in monotonic and cyclic tests, for three loading orientations. The ECAP-ed samples demonstrate: (a) a good grain refinement from 50 - 70 μm down to 2.5 - 7 μm), (b) a superplastic ZK60 alloy, with an elongation to failure of 371 % at 3500C and the strain rate of 10-2 s-1, and (c) a longer fatigue life for the AZ31 alloy, relative to the as-received material. The starting and ECAP-deformed materials were characterized by optical microscopy, Xray diffraction using both soft and hard X-rays, and neutron diffraction. The grain sizes, the textures, the coherent-domain sizes, the elastic microstrains, and the dislocation densities were determined for the samples deformed by rolling, extrusion, and ECAP. The synchrotron radiation measurements allowed monitoring the lattice rotation induced by the ECAP deformation in Mg alloys. The grain-orientation dependent deformation is studied relative to the deformation history, and its influence on the mechanical behavior is analyzed relative to the twinning contribution. The results of the present work constitute a valuable benchmark for the understanding and modeling of the deformation mechanisms, such as the dislocations slip, twinning, recovery, or recrystallization in HCP structures

Fatigue characteristics and modeling of cast and cast-forged ZK60 magnesium alloy

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.ijfatigue.2018.03.019 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The fatigue behavior of as-cast and cast-forged ZK60 magnesium alloy was investigated via fully-reversed strain controlled fatigue tests at different strain amplitudes. Microstructure analysis, texture measurement, and SEM fracture surface characterization were performed to discern the reason of fatigue behavior improvement via forging, and also to explain the mechanism underlying crack initiation in both cast and cast-forged conditions. It was perceived that the forged alloy contains less amount of porosities and second phase particles in its microstructure. In general, the forged alloy showed longer fatigue life for all strain amplitudes, especially when the strain amplitude is lower than 0.4%. The forging process increased the fatigue strength at 107 cycles from 0.175% to 0.22% strain amplitude. The microstructure obtained after fatigue test showed that twinning can be activated in the cast-forged alloy, once strain amplitude is higher than 0.4%. The interaction of twin bands with the grain boundaries can also adversely affect the fatigue life of the forged alloy. Also, the residual twins can develop tensile mean stress which affects the fatigue life negatively. Finally, the Coffin-Manson fatigue model and an energy-based fatigue model were employed to model the life of as-cast and cast-forged materials. While some of the predicted lives by the former were out of the ±2× boundary bounds, the latter’s results were tightly clustered in ±1.5× bounds.Natural Sciences and Engineering Research Council of CanadaAutomotive Partnership Canada ["APCPJ 459269-13"

Role of Loading Direction on Fatigue Behavior of Smooth and Notched ZK60 Extrusion

In spite of the increasing trend of magnesium (Mg) application over the last decade, numerous technical restrictions still limit Mg alloy’s extensive implementation in industry. For instance, many mechanical behaviors of Mg alloys are still unknown, which make them risky options for manufacturers who prefer to be well-informed about raw materials’ characteristics so as to maximize their product reliability. Therefore, a comprehensive characterization in conjunction with life assessment via empirical models is needed to expand these alloys’ application. A literature review has revealed the potential of energy-based models to be employed in strain- and stress-controlled modeling of wrought alloys. However, to the best of the author’s knowledge, no study is available yet on the anisotropic fatigue behavior and modeling of smooth and notched ZK60 extrusion, which is the focal point of this research. The quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along three different directions: the extrusion direction (ED), the radial direction (RD), and 45° to the extrusion direction (45°). The quasi-static response showed symmetric behavior for the samples tested along RD and 45°, whereas ED samples manifested completely asymmetric behavior. Although the ED samples exhibited longer fatigue lives than the RD and 45° in the high cycle fatigue, the fatigue lives in the low cycle fatigue regime were similar. Microstructural analysis revealed finer grains for ED, thereby higher strength for this direction. Moreover, the texture measurement indicated a sharp basal texture justifying asymmetric behavior solely along the ED direction. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for showing identical fatigue responses to those of RD and 45° in the low cycle fatigue regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twined grains, whereas slip bands were dominant on the fracture surface in RD direction. Smith-Watson-Topper, as a critical-plane strain-based criterion, and Jahed-Varvani (JV), as an energy-based damage criterion, were employed to predict the strain-controlled fatigue lives along all directions using a single set of material parameters. Fully reversed stress-control condition was also investigated over a wide range of stress amplitudes along two different material directions: ED and RD. The in-plane random texture along RD promotes activation of twinning/detwinning deformations in both tension and compression reversals, which brings about a sigmoidal but near-symmetric shape for hysteresis loops along this direction. The stress-strain response along ED is asymmetric in tension and compression reversals if subjected to high-stress amplitudes. The asymmetry is attributed to different deformation mechanisms being active in the tension and compression reversals. Overall, loading along ED yields superior fatigue performance compared to RD. One set of JV coefficients extracted from the strain-controlled tests was employed and successfully modeled the anisotropic stress-control fatigue response of the material. The effect of a material’s orientation on the fatigue response of notched ZK60 extrusion was investigated via fully reversed stress-controlled experiments in ED and RD. The anisotropy observed in the stress-life curves of the notched ED and RD samples resembles the stress-controlled fatigue performance of the smooth samples. This observation is attributed to the higher strength of ED specimens, which imposes less plastic-induced-damage in the stress-controlled fatigue experiments. The ED specimens exhibit not only superior fatigue performance but also higher notch sensitivity while loading along RD significantly reduces the notch sensitivity

Fatigue of Magnesium-Based Materials

Magnesium alloys and metal matrix composites (MMCs) are attractive materials for biomedical application. Magnesium has a module of elasticity that is close to that of human bones and it is biocompatible with the human body. Human body fluids make a corrosive environment to magnesium. In addition, different body parts are subjected to cyclic loading reaching a magnitude of about 80 MPa and an estimated total of 106 cycles per year. Therefore, understanding the fatigue behavior of magnesium alloys and magnesium metal matrix composites (MMCs) is an essential aspect especially when they are used as load bearing components. Magnesium has a hexagonal closed-packed (HCP) lattice structure with a c/a ratio of 1.623, and it does not have enough independent slip systems to sustain large plastic deformation. Therefore, magnesium deforms plastically by two different mechanisms: slipping and twinning. Twinning-detwinning deformation is manifested in the cyclic stress-strain response of wrought magnesium alloys when loaded along the working direction. A significant stress asymmetry is usually observed resulting in the development of high mean stress. Research on magnesium and its alloys is rapidly increasing. This chapter presents different aspects of fatigue, in general, and on magnesium in particular, including experimental method, damage models and fatigue life equation

Anisotropy in the quasi-static and cyclic behavior of ZK60 extrusion: Characterization and fatigue modeling

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.matdes.2018.10.026 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along three different directions: the extrusion direction (ED), the radial direction (RD), and 45° to the extrusion direction (45°). The quasi-static response showed symmetric behavior for the samples tested along RD and 45°, whereas the ED samples manifested completely asymmetric behavior. Although the ED samples exhibited longer fatigue lives than the RD and 45° in the high cycle fatigue, the fatigue lives in the low cycle fatigue regime were similar. The texture measurement indicated a sharp basal texture along ED, explaining its asymmetric behavior. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for exhibiting fatigue responses identical to those of RD and 45° in the LCF regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twinned grains, whereas that in RD was dominated by slip bands. Finally, Smith-Watson-Topper and Jahed-Varvani models were employed to predict the fatigue lives along all directions using a single set of material parameters.Natural Sciences and Engineering Research Council of CanadaAssociation for Progressive Communications ["459269–13"